1. Field of the Invention
The present invention relates to noise filtering circuits for suppressing electromagnetic interference (EMI), and more particularly, to a noise filtering circuit for filtering out a high multiplied-frequency component of a high-speed digital signal within a broadband frequency band and enabling transmission of a passband of the digital signal.
2. Description of the Prior Art
The operation speed and clock frequency of a high-speed digital circuit increases with the rapid development of consumer electronic product. The faster a digital signal is, the broader the distribution of its frequency domain components is. However, in practical electronic circuit design, asymmetry is inevitable. For example, to minimize the occupied area of a delay line, a meandered line is arranged in the form of bending, penetrating a via, or becoming discrete in order to give way to a via. Regarding the meandered lines, electromagnetic interference (EMI) usually arises from the high multiplied-frequency component of a signal passing through the discrete structures or being sent to a connected wire or metallic shield, and as a result issues pertaining to severe electromagnetic compatibility and EMI confront related products.
In view of this, some methods for filtering out a high multiplied-frequency component of a signal were put forth. Among the methods, the most typical one involves ferrite beads/toroid choke, that is, blocking the high multiplied-frequency component of the signal by the high inductance of a ferromagnet. However, due to rapid attenuation of the magnetic permeability of the ferromagnet at a high frequency, the ferrite beads/toroid choke are actually inapplicable to a high-frequency signal operating at GHz-level frequencies. Furthermore, due to its large dimensions, the aforesaid structure is inapplicable to a high-speed digital circuit.
Surface mount devices (SMD) available on an industrial scale nowadays are miniaturized structures designed in accordance with the prototype of a T-section or pi-section low-pass filter. However, the miniaturized filter structures require large series-connected inductance and bypass capacitance, as far as their design is concerned, thereby incurring high design costs.
Accordingly, it is imperative to provide a noise filtering circuit conducive to elimination of the aforesaid drawbacks of the prior art, such as narrow application and high design costs.